This invention generally relates to wireless communication systems. In particular, the invention relates to transmission of data in such systems where adaptive modulation & coding (AMC) and hybrid automatic repeat request (H-ARQ) techniques are applied.
In wireless communication systems, such as the third generation partnership project (3GPP) time division duplex (TDD) or frequency division duplex (FDD) communication systems using code division multiple access (CDMA) or orthogonal frequency division multiplex (OFDM) systems, AMC is used to optimize the use of air resources.
The modulation and coding schemes (sets) used to transmit data are varied based on wireless channel conditions. To illustrate, a type of error encoding (such as turbo versus convolutional coding), coding rate, spreading factor for CDMA system, modulation type (such as quadrature phase shift keying versus M-ary quadrature amplitude modulation), and/or adding/subtracting sub-carriers for an OFDM system may change. If channel characteristics improve, a lower data redundancy and/or “less robust” modulation and coding set is used to transfer data. As a result, for a given allocation of radio resources, more user data is transferred resulting in a higher effective data rate. Conversely, if channel characteristics degrade, a higher data redundancy “more robust” modulation and coding set is used, transferring less user data. Using AMC, an optimization between air resource utilization and quality of service (QOS) can be better maintained.
Data in such systems is received for transfer over the air interface in transmission time intervals (TTIs). Data within a TTI transferred to a particular user equipment is referred to as a transport block set (TBS). For a particular allocation of air resources, a less robust modulation and coding set allows for larger TBS sizes and a more robust modulation and coding set only allows for smaller TBS sizes. As a result, the modulation and coding set for a given radio resource allocation dictates the maximum size of the TBS that can be supported in a given TTI.
In such systems, a hybrid automatic repeat request (H-ARQ) mechanism may be used to maintain QOS and improve radio resource efficiency. A system using H-ARQ is shown in FIG. 1. A transmitter 20 transmits a TBS over the air interface using a particular modulation and coding set. The TBS is received by a receiver 26. An H-ARQ decoder 30 decodes the received TBS. If the quality of the received data is unacceptable, an ARQ transmitter 28 requests a retransmission of the TBS. One approach to check the quality of the received TBS is a cyclic redundancy check (CRC). An ARQ receiver 22 receives the request and a retransmission of the TBS is made by the transmitter 20. Retransmissions may apply a more robust modulation and coding set to increase the possibility of successful delivery. The H-ARQ decoder 30 combines, the received TBS versions. A requirement for combining is that combined TBSs are identical. If the resulting quality is still insufficient, another retransmission is requested. If the resulting quality is sufficient, such as the combined TBS passes the CRC check, the received TBS is released for further processing. The H-ARQ mechanism allows for data received with unacceptable quality to be retransmitted to maintain the desired QOS.
In a system using both H-ARQ and AMC, a change in modulation and coding set may be determined necessary to achieve successful delivery of a requested TBS retransmission. In this situation, the maximum amount of physical data bits allowed within the TTI varies with the modulation and coding set.
Since only one TBS exists per TTI the effective user data rate corresponds to the TBS size applied to each TTI. To achieve maximum data rates the largest TBS size is applied to the least robust modulation and coding set within the TTI. When wireless channel conditions require a more robust modulation and coding set for successful transmission, such as when a TBS size can not be supported within the TTI. Therefore, when operating at the maximum data rate, each time a more robust modulation and coding requirement is realized, all outstanding transmissions in H-ARQ processes that have not been successfully acknowledged must be discarded.
When Incremental Redundancy (IR) is applied, TBS data must remain constant in retransmissions for proper combining. Therefore, to guarantee that a TBS retransmission can be supported at a more robust modulation and coding set then the initial transmission, the TBS size used must correspond to the most robust MCS. However, when a TBS size allowed by the most robust modulation and coding set is appliedthe maximum data rate to the mobile is reduced, and when a less robust modulation and coding set is applied physical resources are not fully utilized.
When the TBS size is not supported by the more robust modulation and coding set, the TBS can be retransmitted using the old modulation and coding set. However, if the channel conditions dictate that a more robust modulation and coding set be used or the initial transmission was severally corrupted, the combining of the retransmitted TBSs may never pass, resulting in a transmission failure.
In current implementations, when a TBS can not be successfully transmitted by AMC & H-ARQ mechanisms, recovery is handled by the radio link control (RLC) protocol (at layer two). Unlike a H-ARQ recovery of failed transmissions, the RLC error detection, data recovery and buffering of a TBS queued in the node-B, results in increased block error rates and data latency, potentially resulting in a failure to meet QOS requirements.
Accordingly, to provide maximum data rates with minimal H-ARQ transmission failures, it is desirable to support incremental redundancy and allow adaptation of modulation and coding sets in such systems.